1. Field of the Invention
The present invention relates to a fuel cell stack comprising a plurality of power generation units each including at least first and second electrolyte electrode assemblies. The first electrolyte electrode assembly is stacked on a first separator, a second separator is stacked on the first electrolyte electrode assembly, the second electrolyte electrode assembly is stacked on the second separator, and a third separator is stacked on the second electrolyte electrode assembly. Each of the first and second electrolyte electrode assemblies includes a pair of electrodes and an electrolyte interposed between the electrodes.
2. Description of the Related Art
For example, a polymer electrolyte fuel cell employs a membrane electrode assembly which includes an anode, a cathode, and an electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly and separators sandwiching the membrane electrode assembly make up a unit of a power generation cell for generating electricity. Normally, a predetermined numbers of the power generation cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas flow field (reactant gas flow field) for supplying a fuel gas along the anode and an oxygen-containing gas flow field (reactant gas flow field) for supplying an oxygen-containing gas along the cathode are provided. Further, a coolant flow field for supplying a coolant along separator surfaces is provided between the separators.
In some of power generation cells of the fuel cell stack, in comparison with the other power generation cells, the temperature is decreased easily due to heat radiation to the outside. For example, in the power generation cells provided at ends of the fuel cell stack in the stacking direction (hereinafter also referred to as the “end power generation cells”), since the heat is radiated to the outside from the terminal plates (current collecting plates) for collecting electrical charges generated in each of the power generation cells as electricity or the end plates for tightening the stacked power generation cells, the decrease in the temperature is significant.
Therefore, due to the decrease in the temperature, in the end power generation cells, in comparison with power generation cells in the central position of the fuel cell stack, water condensation occurs easily, and the water produced in the power generation cannot be discharged smoothly. Consequently, the power generation performance of the end power generation cells is low.
In this regard, for example, a fuel cell stack disclosed Japanese Laid-Open Patent Publication No. 2006-147502 is known. The fuel cell stack includes a stack body formed by stacking a plurality of power generation cells, and a dummy cell provided at least at one end of the stack body in the stacking direction. The dummy cell has a dummy electrode body including an electrically conductive plate corresponding to an electrolyte, and dummy separators sandwiching the dummy electrode body. The dummy separators have structure identical to normal separators.
In this case, the dummy cell does not use any electrolyte. Therefore, no power generation occurs in the dummy cell, and water is not produced in the dummy cell. Thus, the dummy cell itself functions as a heat insulating layer. At the time of starting operation of the fuel cell stack at low temperature, it is possible to effectively prevent the delay in the raise of temperature in the end power generation cell, and the voltage drop in the end power generation cell.
In the fuel cell stack, the coolant flow field is provided at intervals of a certain number of power generation cells for so called skip cooling. That is, in the design, the number of the coolant flow fields is decreased to reduce the overall size of the fuel cell stack in the stacking direction. In the fuel cell stack adopting the skip cooling structure of this type, it is desired to effectively prevent the delay in the raise of temperature, and voltage drop of the end power generation cell.